Temporarily Bound Prosthetic Intervertebral Discs Implantable By Minimally Invasive Surgical Techniques

ABSTRACT

The described devices are bound spinal implants that may be surgically implanted into the spine to replace damaged or diseased discs using a posterior approach. The discs are prosthetic devices that approach or mimic the physiological motion and reaction of the natural disc.

RELATED APPLICATIONS

This application claims benefit from U.S. provisional patent applicationNo.60/909,454, filed Mar. 31, 2007, the entirety of which isincorporated by reference.

FIELD

The described devices are bound spinal implants that may be surgicallyimplanted into the spine to replace damaged or diseased discs using aposterior approach. The discs are prosthetic devices that approach ormimic the physiological motion and reaction of the natural disc.

BACKGROUND

The intervertebral disc is an anatomically and functionally complexjoint. The intervertebral disc is composed of three componentstructures: (1) the nucleus pulposus; (2) the annulus fibrosus; and (3)the vertebral end plates. The biomedical composition and anatomicalarrangements within these component structures are related to thebiomechanical function of the disc.

The spinal disc may be displaced or damaged due to trauma or a diseaseprocess. If displacement or damage occurs, the nucleus pulposus mayherniate and protrude into the vertebral canal or intervertebralforamen. Such deformation is known as herniated or slipped disc. Aherniated or slipped disc may press upon the spinal nerve that exits thevertebral canal through the partially obstructed foramen, causing painor paralysis in the area of its distribution.

To alleviate this condition, it may be necessary to remove the involveddisc surgically and fuse the two adjacent vertebrae. In this procedure,a spacer is inserted in the place originally occupied by the disc andthe spacer is secured between the neighboring vertebrae by the screwsand plates or rods attached to the vertebrae. Despite the excellentshort-term results of such a “spinal fusion” for traumatic anddegenerative spinal disorders, long-term studies have shown thatalteration of the biomechanical environment leads to degenerativechanges particularly at adjacent mobile segments. The adjacent discshave increased motion and stress due to the increased stiffness of thefused segment. In the long term, this change in the mechanics of themotion of the spine causes these adjacent discs to degenerate.

Artificial intervertebral replacement discs may be used as analternative to spinal fusion.

SUMMARY

Prosthetic intervertebral discs and methods for using such discs aredescribed. The subject prosthetic discs include an upper end plate, alower end plate, and a compressible core member disposed between the twoend plates. The described prosthetic discs have shapes, sizes, and otherfeatures that are particularly suited for implantation using minimallyinvasive surgical procedures, particularly from a posterior approach. Inparticular, the described discs are releasably bound with an elongatemember, e.g., string, cable, wire, etc., to allow remotely controlledexpansion of a bound disc.

The described bound prosthetic discs include top and bottom end platesseparated by one or more compressible core members. I also refer to thetop and bottom end plates as first and second end plates, particularlysince the nature of this device is such that it operates the same waywithout regard to the orientation of the device. The two plates may beheld together by at least one fiber wound around at least one region ofthe top end plate and at least one region of the bottom end plate. Thedescribed discs may include integrated vertebral body fixation elements.When considering a lumbar disc replacement from the posterior access,the two plates are preferably elongated, having a length that issubstantially greater than its width. Typically, the dimensions of theprosthetic discs range in height from 8 mm to 15 mm; the width rangesfrom 6 mm to 13 mm. The height of the prosthetic discs ranges from 9 mmto 11 mm. The widths of the disc may be 10 mm to 12 mm. The length ofthe prosthetic discs may range from 18 mm to 30 mm, perhaps 24 mm to 28mm. Typical shapes include oblong, bullet-shaped, lozenge-shaped,rectangular, or the like.

The described bound disc structures may be held together by at least onefiber wound around at least one region of the upper end plate and atleast one region of the lower end plate. The fibers are generally hightenacity fibers with a high modulus of elasticity. The elasticproperties of the fibers, as well as factors such as the number offibers used, the thickness of the fibers, the number of layers of fiberwindings in the disc, the tension applied to each layer, and thecrossing pattern of the fiber windings enable the prosthetic discstructure to mimic the functional characteristics and biomechanics of anormal-functioning, natural disc.

A number of conventional surgical approaches may be used to place a pairof prosthetic discs. Those approaches include a modified posteriorlumbar interbody fusion (PLIF) and a modified transforaminal lumbarinterbody fusion (TLIF) procedures. We also describe apparatus andmethods for implanting prosthetic intervertebral discs using minimallyinvasive surgical procedures. In one variation, the apparatus includes apair of cannulae that are inserted posteriorly, side-by-side, to gainaccess to the spinal column at the disc space. A pair of prostheticdiscs may then be implanted by way of the cannulae to be located betweentwo vertebral bodies in the spinal column.

The prosthetic discs may be configured by selection of sizes andstructures suitable for implantation by minimally invasive procedures.The discs may be bound, perhaps in a compressed condition or specificform, by one or more elongate members, e.g., string, cable, wire, etc.that may tied to remotely allow release of the disc.

Other and additional devices, apparatus, structures, and methods aredescribed by reference to the drawings and detailed descriptions below.

BRIEF DESCRIPTION OF THE DRAWINGS

The Figures contained herein are not necessarily drawn to scale. Somecomponents and features may be exaggerated for clarity.

FIG. 1 shows a method for placement of prosthetic intervertebral discsusing a posterior approach.

FIG. 2A is a side view of one variation of my bound prosthetic disc.

FIG. 2B is a perspective view of the variation seen in FIG. 2A.

FIG. 3 is a side view of another variation of my bound prosthetic disc.

FIG. 4 is a side view of another variation of my bound prosthetic disc.

FIG. 5 is a perspective view of another variation of my bound prostheticdisc.

FIG. 6 schematically illustrates a method for implanting the describedprosthetic discs.

DETAILED DESCRIPTION

Described below are prosthetic intervertebral discs, methods of usingsuch discs, apparatus for implanting such discs, and methods forimplanting such discs. It is to be understood that the prostheticintervertebral discs, implantation apparatus, and methods are notlimited to the particular embodiments described, as these may, ofcourse, vary. It is also to be understood that the terminology used hereis only for the purpose of describing particular embodiments, and is notintended to be limiting in any way.

Insertion of the prosthetic discs may be approached using modifiedconventional procedures, such as a posterior lumbar interbody fusion(PLIF) or transforaminal lumbar interbody fusion (TLIF). In the modifiedPLIF procedure, the spine is approached via midline incision in theback. The erector spinae muscles are stripped bilaterally from thevertebral lamina at the required levels. A laminectomy is then performedto further allow visualization of the nerve roots. A partial facetectomymay also be performed to facilitate exposure. The nerve roots areretracted to one side and a discectomy is performed. Optionally, achisel may then used to cut one or more grooves in the vertebral endplates to accept the fixation components on the prostheses.Appropriately-sized prostheses may then be inserted into theintervertebral space on either side of the vertebral canal.

In a modified TLIF procedure, the approach is also posterior, butdiffers from the PLIF procedure in that an entire facet joint is removedand the access is only on one side of the vertebral body. After thefacetectomy, the discectomy is performed. Again, a chisel may be used tocreate on or more grooves in the vertebral end plates to cooperativelyaccept the fixation components located on each prosthesis. Theprosthetic discs may then be inserted into the intervertebral space. Oneprosthesis may be moved to the contralateral side of the access and thena second prosthesis then inserted on the access side.

It should be apparent that we refer to these procedures as “modified” inthat neither procedure is used to “fuse” the two adjacent vertebrae.

FIG. 1 shows a top, cross section view of a spine (100), sectionedacross an intervertebral disc (102). This Figure depicts a minimallyinvasive surgical procedure for implanting a pair of intervertebraldiscs in an intervertebral region formed by the removal of a naturaldisc. This minimally invasive surgical implantation method is performedusing a posterior approach, rather than the conventional anterior lumbardisc replacement surgery or the modified PLIF and TLIF proceduresdescribed above.

In FIG. 1, two cannulae (104) are inserted posteriorly, through the skin(107), to provide access to the spinal column. More particularly, asmall incision is made and a pair of access windows created through thelamina (106) of one of the vertebrae (108) on each side of the vertebralcanal (110) to access the natural vertebral disc. The spinal cord (112)and nerve roots are avoided or moved to provide access. Once access isobtained, the two cannulae (104) are inserted. The cannulae (104) may beused as access passageways in removing the natural disc withconventional surgical tools. Alternatively, the natural disc may beremoved prior to insertion of the cannulae. The cannulae are also usedto introduce the prosthetic intervertebral discs (114) to theintervertebral region.

The described bound prosthetic discs are of a design and capability suchthat they may be employed at more than one level, i.e., disc location,in the spine. Specifically, several natural discs may be replaced withmy discs. As will be described in greater detail below, each such levelwill be implanted with at least two of my discs. Kits, containing two ofmy discs for a single disc replacement or four of my discs forreplacement of discs at two levels in the spine, perhaps with sterilepackaging are contemplated. Such kits may also contain one or morecannulae having a central opening allowing passage and implantation ofmy discs.

Once the natural disc has been removed and the cannulae (104) located inplace, a pair of prosthetic discs (114) is implanted between adjacentvertebral bodies. The prosthetic discs have a shape and size suitablemaking them useful with (or adapted for) various minimally invasiveprocedures. The discs may have a shape such as the elongated one-pieceprosthetic discs described below.

A prosthetic disc (114) is guided through each of the cannula such thateach of the prosthetic discs (114) is implanted between the two adjacentvertebral bodies. The two prosthetic discs (114) may be locatedside-by-side and spaced slightly apart, as viewed from above.Optionally, prior to implantation, grooves may be formed on the internalsurfaces of one or both of the vertebral bodies in order to engageanchoring components or features located on or integral with theprosthetic discs (114). The grooves may be formed using a chisel tooladapted for use with the minimally invasive procedure, i.e., adapted toextend through a relatively small access space (such as the tunnel-likeopening found in through the cannulae) and to chisel the noted grooveswithin the intervertebral space present after removal of the naturaldisc.

These discs may be used as shown in FIG. 1 or, optionally, they may beimplanted with an additional prosthetic disc or discs, perhaps in theposition shown for auxiliary disc (116).

Additional prosthetic discs may also be implanted in order to obtaindesired performance characteristics, and the implanted discs may beimplanted in a variety of different relative orientations within theintervertebral space. In addition, the multiple prosthetic discs mayeach have different performance characteristics. For example, aprosthetic disc to be implanted in the central portion of theintervertebral space may be configured to be more resistant tocompression than one or more prosthetic discs that are implanted nearerthe outer edge of the intervertebral space. For instance, the stiffnessof the outer discs (e.g., 114) may each be configured such that thoseouter discs exhibit approximately 5% to 80% of the stiffness of thecentral disc (116), perhaps in the range of about 30% to 60% of thecentral disc (116) stiffness. Other performance characteristics may bevaried as well.

This description may describe a number of variations of prostheticintervertebral discs. By “prosthetic intervertebral disc” is meant anartificial or manmade device that is so configured or shaped that it maybe employed as a total or partial replacement of an intervertebral discin the spine of a vertebrate organism, e.g., a mammal, such as a human.The described prosthetic intervertebral discs have dimensions thatpermit them, either alone or in combination with one or more otherprosthetic discs, to substantially occupy the space between two adjacentvertebral bodies that is present when the naturally occurring discbetween the two adjacent bodies is removed, i.e., a void disc space. By“substantially occupy” is meant that, in the aggregate, the discs occupyat least about 30% by surface area, perhaps at least about 80% bysurface area or more. The subject discs may have a roughly bullet orlozenge shaped structure adapted to facilitate implantation by minimallyinvasive surgical procedures.

The discs may include both an upper (or top) and lower (or bottom) endplate, where the upper and lower end plates are separated from eachother by a compressible element such as one or more core members, wherethe combination structure of the end plates and compressible elementprovides a prosthetic disc that functionally approaches or closelymimics a natural disc. The top and bottom end plates may be heldtogether by at least one fiber attached to or wound around at least oneportion of each of the top and bottom end plates. As such, the two endplates (or planar substrates) are held to each other by one or morefibers that are attached to or wrapped around at least one domain,portion, or area of the upper end plate and lower end plate such thatthe plates are joined to each other.

FIG. 2 shows a variation of my bound prosthetic intervertebral disc(200). This variation comprises an upper end plate (202) and a lower endplate (204) separated by a compressible core (206). As discussed belowin more detail, the compressible core (206) may comprise one or morecore members (not shown) and be bounded by one or more fibers (207)extending between the upper end plate (202) and the lower end plate(204). The upper and lower end plates (202, 204) may include apertures(not shown), through which the fibers (207) may pass. Other components(woven or nonwoven fabrics, wires, etc.) may be used in functionalsubstitution for the fibers (207).

Also included in this variation is a binding element (210) that passesfrom the proximal end (212), through a passageway (214) inset into lowerend plate (204), out the distal end (216), through an opening (218), andback through the passageway (214) to the starting point. When both endsof the binding element (210) are held taut, the distal end (216) of thedisc (200) is compressed. Release of the binding element and itswithdrawal through the pathway, releases the compression of the disc andallows it to expand to its normal height.

FIG. 2B shows a perspective view of the disc shown in FIG. 2A. Thepathway of the binding element (210) through the disc (200) is shown (inshadow)—passing back and forth through the passageway (214) and throughthe distal opening (218).

The utility of the binding element shown in the various incarnationsdescribed here is multiple. First, the profile (or height) of the discis minimized for enhancing the ability of the user to easily andaccurately place the prosthetic disc in the prepared intervertebralspace. Some commercial prosthetic discs are not at all compressible, afactor that limits those discs' suitability for many situations. Thelower profile is better able to pass through small access points intothe intervertebral space. This allows the physician to utilize accesses,e.g., through spinal ligaments, that are smaller. The narrow form factorof the described discs combined with the binding element allows, in somevariations, the physician to introduce the prosthetic disc “vertically”through a narrower ligament opening and then to rotate it into placementposition. The user's ability to situate the bound prosthetic disc in theintervertebral space with a simple grasping tool and then to release thebinding element by a simple pull to finally place the disc in its finalimplantation site is significant. The ease of “tool use” is a verypositive attribute. The form of the binding element may be tailored forspecific situations. In particular, the binding element may be attachedin such a way that only one end of the prosthetic disc is compressed,e.g., see FIG. 2A, 2B, and 3. The binding element may be attached toprovide differential compression, e.g., wherein one end of the disc iscompressed more than is the other.

FIG. 3 is a side view of another variation of my device (250) showingthe binding element (252). The binding element (252) passes through apair of openings (254) in the distal end of the upper and lower endplates (202, 204). This arrangement provides an angle between those endplates.

FIG. 4 is a side view of another variation of my disc (260), in thisinstance providing an example of a binding element (260) around the discand extending from the proximal end (264) of the disc to the distal end(266) of the disc and then back. This arrangement can provide fullcompression to the disc, both in the sense that the disc is able toachieve the lowest possible height and also that the compression isachievable along the complete length (distal-proximal) of the disc.

FIG. 5 shows a further variation of my bound prosthetic disc (270)having a binding element (272) that envelopes the disc in two axes, fromside-to-side and from distal to proximal. In this and in all of thevariations shown herein, the designer may choose to use more than onebinding element on a single disc. For instance, the variations shown inFIGS. 2A, 2B, and 3 may also include an additional, separate bindingelement at the other non-bound end so that the two binding elements maybe separately released allowing differential expansion (distal-proximal)for even more control during placement in the intervertebral space.

The binding element may be made from a wide variety of materials.Suitable materials include synthetic polymers, natural polymers (e.g.,silk), metals, alloys (including superelastic alloys such as nitinol),coated materials (e.g., lubricious polymers on “strength” polymers,lubricious polymers on metals or alloys, etc.), and the like. The formof the binding element may be a monofilament, thread or multifilament,cable, etc.

The release mechanism for each of these variations may be any of avariety of slip knots (with safety lines, if desired), crimpings,twisted wire, etc.

The end plates may be planar substrates having a length of from about 12mm to about 45 mm, such as from about 13 mm to about 44 mm, a width offrom about 11 mm to about 28 mm, such as from about 12 mm to about 25mm, and a thickness of from about 0.5 mm to about 5 mm, such as fromabout 1 mm to about 3 mm. The top and bottom end plates are fabricatedor formed from a physiologically acceptable material that provides forthe requisite mechanical properties, primarily structural rigidity anddurability. Representative materials from which the end plates may befabricated are known to those of skill in the art and include: metalssuch as titanium, titanium alloys, stainless steel, cobalt/chromium,etc.; plastics such as polyethylene with ultra high molar mass(molecular weight) (UHMW-PE), polyether ether ketone (PEEK), etc.;ceramics; graphite; etc.

The discs may also include fibers wound between and connecting the upperand lower end plates. These fibers may extend through a plurality ofopenings or apertures formed on portions of each of the upper and lowerend plates. Thus, a fiber extends between the pair of end plates, andextends up through an aperture in the upper end plate and back downthrough an adjacent aperture in that end plate. The fibers need not betightly wound, thereby allowing a degree of axial rotation, bending,flexion, and extension by and between the end plates. The amount ofaxial rotation generally is in the range from about 0° to about 15°,perhaps from about 2° to 10°. The amount of bending generally has arange from about 0° to about 18°, perhaps from about 2° to 15°. Theamount of flexion and extension generally has a range from about 0° toabout 25°, perhaps from about 3° to 15°. Of course, the fibers may bemore or less tightly wound to vary the resultant values of theserotational values. The core members forming compressible core may beprovided in an uncompressed or in a compressed state. An annular capsulemay be included in the space between the upper and lower end platessurrounding the compressible core.

My described prosthetic discs may include a compressible core comprisinga larger single elongated core member, a generally circular core member,or two or more generally cylindrical core members. The dual corestructure may better simulate the performance characteristics of anatural disc. In addition, the fibers found in the dual core structureare believed to endure less stress relative to the fibers found in thesingle core structure.

The lateral, or horizontal, surface area of each of the end plates—i.e.,the area of the disc surfaces that engage the vertebral bodies—issubstantially larger than the cross-sectional surface area of the coremember or members. The cross-sectional surface area of the core memberor members may be from about 5% to about 80% of the cross-sectional areaof a given end plate, perhaps from about 10% to about 60%, or from about15% to about 50%. In this way, for a given compressible core havingsufficient compression, flexion, extension, rotation, and otherperformance characteristics but having a relatively smallcross-sectional size, the core member may be used to support end plateshaving a relatively larger cross-sectional size in order to help preventsubsidence into the vertebral body surfaces. In the variations describedhere, the compressible core and end plates also have a size that isappropriate for or adapted for implantation by way of posterior accessor minimally invasive surgical procedures, such as those describedabove.

FIG. 6, step (a), shows the placement of a compressed disc (300) intothe intervertebral space (302) between an upper vertebra (304) and theadjacent lower vertebra (306). No placement tool is shown, for ease ofexplanation. The compressed disc (300) has been passed through thecannula (310) to the implantation site with a binding element (312) inplace.

FIG. 6, step (b), shows the disc (300) after removal of the bindingelement (312) and expansion of the disc. The cannula (310) and thebinding element (312) are shown being removed.

Each of the described prosthetic discs depicted in the Figures has agreater length than width. The aspect ratio (length:width) of the discsmay be about 1.5:1 to 5.0:1, perhaps about 2.0:1 to 4.0:1, or about2.5:1 to 3.5:1. Exemplary shapes to provide these relative dimensionsinclude rectangular, oval, bullet-shaped, lozenge-shaped, and others.These shapes facilitate implantation of the discs by the minimallyinvasive procedures described above. Of course, the shape of my discs isnot limited to those listed just above, but may be any shape suitablefor use as all or part of a complete intervertebral prosthetic disc.

The surfaces of the upper and lower end plates, i.e., those surfaces incontact with and eventually adherent to the respective opposed bonysurfaces of the upper and lower vertebral bodies, may have one or moreanchoring or fixation components or mechanism for securing those endplates to the vertebral bodies. For example, the anchoring feature maybe one or more “keels,” a fin-like extension often having asubstantially triangular cross-section and having a sequence of exteriorbarbs or serrations. This anchoring component is intended tocooperatively engage a mating groove that is formed on the surface ofthe vertebral body and to thereby secure the end plate to its respectivevertebral body. The serrations enhance the ability of the anchoringfeature to engage the vertebral body.

Further, this variation of the anchoring component may include one ormore holes, slots, ridges, grooves, indentations, or raised surfaces tofurther assist in anchoring the disc to the associated vertebra. Thesephysical features will so assist by allowing for bony ingrowth. Each endplate may have a different number of anchoring components, and thoseanchoring features may have a different orientation on each end plate.The number of anchoring features generally ranges in number from about 0to about 500, perhaps from about 1 to 10. Alternatively, anotherfixation or anchoring mechanism may be used, such as ridges, knurledsurfaces, serrations, or the like. In some variations, the discs willhave no external fixation mechanism. In such variations, the discs areheld in place laterally by the friction forces between the disc and thevertebral bodies.

Further, each of the described variations may additionally include aporous covering or layer (e.g., sprayed Ti metal) allowing boneyingrowth and may include some osteogenic materials.

As noted above, in the variations shown herein, the upper end plate andlower end plate may each contain a plurality of apertures through whichthe fibers may be passed through or wound, as shown. The actual numberof apertures contained on an end plate is variable. Increasing thenumber of apertures allows an increase in the circumferential density ofthe fibers holding the end plates together. The number of apertures mayrange from about 3 to 100, perhaps in the range of 10 to 30. Inaddition, the shape of the apertures may be selected so as to provide avariable width along the length of the aperture. For example, the widthof the apertures may taper from a wider inner end to a narrow outer end,or visa versa. Additionally, the fibers may be wound multiple timeswithin the same aperture, thereby increasing the radial density of thefibers. In each case, this improves the wear resistance and increasesthe torsional and flexural stiffness of the prosthetic disc, therebyfurther approximating natural disc stiffness. In addition, the fibersmay be passed through or wound on each aperture, or only on selectedapertures, as needed. The fibers may be wound in a uni-directionalmanner, where the fibers are wound in the same direction, e.g.,clockwise, which closely mimics natural annular fibers found in anatural disc, or the fibers may be wound bi-directionally. Other windingpatterns, both single and multi-directional, may also be used.

The apertures provided in the various end plates discussed here, may beof a number of shapes. Such aperture shapes include slots with constantwidth, slots with varying width, openings that are substantially round,oval, square, rectangular, etc. Elongated apertures may be radiallysituated, circumferentially situated, spirally located, or combinationsof these shapes. More than one shape may be utilized in a single endplate.

One purpose of the fibers is to hold the upper and lower end platestogether and to limit the range-of-motion to mimic or at least toapproach the range-of-motion of a natural disc. The fibers may comprisehigh tenacity fibers having a high modulus of elasticity, for example,at least about 100 MPa, perhaps at least about 500 MPa. By high tenacityfibers is meant fibers able to withstand a longitudinal stress of atleast 50 MPa, and perhaps at least 250 MPa, without tearing. The fibers207 are generally elongate fibers having a diameter that ranges fromabout 100 μm to about 1000 μm, and preferably about 200 μm to about 400μm. The fibrous components may be single strands or, more typically,multi-strand assemblages. Optionally, the fibers may be injection moldedor otherwise coated with an elastomer to encapsulate the fibers, therebyproviding protection from tissue ingrowth and improving torsional andflexural stiffness. The fibers may be coated with one or more othermaterials to improve fiber stiffness and wear. Additionally, the coremay be injected with a wetting agent such as saline to wet the fibersand facilitate the mimicking of the viscoelastic properties of a naturaldisc. The fibers may comprise a single or multiple component fibers.

The fibers may be fabricated from any suitable material. Examples ofsuitable materials include polyesters (e.g., Dacron® or the Nylons),polyolefins such as polyethylene, polypropylene, low-density and highdensity polyethylenes, linear low-density polyethylene, polybutene, andmixtures and alloys of these polymers. HDPE and UHMWPE are especiallysuitable. Also suitable are various polyaramids, poly-paraphenyleneterephthalamide (e.g., Kevlar®), carbon or glass fibers, variousstainless steels and superelastic alloys (such as nitinol), polyethyleneterephthalate (PET), acrylic polymers, methacrylic polymers,polyurethanes, polyureas, other polyolefins (such as polypropylene andother blends and olefinic copolymers), halogenated polyolefins,polysaccharides, vinylic polymers, polyphosphazene, polysiloxanes,liquid crystal polymers such as those available under the tradenameVECTRA, polyfluorocarbons such as polytetrafluoroethylene and e-PTFE,and the like.

The fibers may be terminated on an end plate in a variety of ways. Forinstance, the fiber may be terminated by tying a knot in the fiber onthe superior or inferior surface of an end plate. Alternatively, thefibers may be terminated on an end plate by slipping the terminal end ofthe fiber into an aperture on an edge of an end plate, similar to themanner in which thread is retained on a thread spool. The aperture mayhold the fiber with a crimp of the aperture structure itself, or by anadditional retainer such as a ferrule crimp. As a further alternative,tab-like crimps may be machined into or welded onto the end platestructure to secure the terminal end of the fiber. The fiber may then beclosed within the crimp to secure it. As a still further alternative, apolymer may be used to secure the fiber to the end plate by welding,including adhesives or thermal bonding. That terminating polymer may beof the same material as the fiber (e.g., UHMWPE, PE, PET, or the othermaterials listed above). Still further, the fiber may be retained on theend plates by crimping a cross-member to the fiber creating a T-joint,or by crimping a ball to the fiber to create a ball joint.

The core members provide support to and maintain the relative spacingbetween the upper and lower end plates. The core members may compriseone or more relatively compliant materials. In particular, thecompressible core members in this variation and the others discussedherein, may comprise a thermoplastic elastomer (TPE) such as apolycarbonate-urethane TPE having, e.g., a Shore value of 50D to 60D,e.g. 55D. An example of such a material is the commercially availableTPE, BIONATE. Shore hardness is often used to specify flexibility orflexural modulus for elastomers.

We have had success with core members comprising TPE that arecompression molded at a moderate temperature from an extruded plug ofthe material. For instance, with the polycarbonate-urethane TPEmentioned above, a selected amount of the polymer is introduced into aclosed mold upon which a substantial pressure may be applied, while heatis applied. The TPE amount is selected to produce a compression memberhaving a specific height. The pressure is applied for 8-15 hours at atemperature of 70-90° C., typically about 12 hours at 80° C.

Other examples of suitable representative elastomeric materials includesilicone, polyurethanes, polybutylene terephthalate (PBT), polybutyleneglycol (polytetramethylene oxide or PMTO), polyesters (e.g., Hytrel®),their mixtures or the like.

Compliant polyurethane elastomers are discussed generally in, M.Szycher, J. Biomater. Appl. “Biostability of polyurethane elastomers: acritical review”, 3(2):297 402 (1988); A. Coury, et al., “Factors andinteractions affecting the performance of polyurethane elastomers inmedical devices”, J. Biomater. Appl. 3(2):130 179 (1988); and Pavlova M,et al., “Biocompatible and biodegradable polyurethane polymers”,Biomaterials 14(13):1024 1029 (1993). Examples of suitable polyurethaneelastomers include aliphatic polyurethanes, segmented polyurethanes,hydrophilic polyurethanes, polyether-urethane, polycarbonate-urethane,and silicone-polyether-urethane.

Other suitable elastomers include various polysiloxanes (or silicones),copolymers of silicone and polyurethane, polyolefins, thermoplasticelastomers (TPE's) such as atactic polypropylene, block copolymers ofstyrene and butadiene (e.g., SBS rubbers), polyisobutylene, andpolyisoprene, neoprene, polynitriles, artificial rubbers such asproduced from copolymers produced of 1-hexene and5-methyl-1,4-hexadiene.

One variant of the construction for the core member comprises a nucleusformed of a hydrogel and an elastomer reinforced fiber annulus.

For example, the nucleus, the central portion of the core member, maycomprise a hydrogel material. Hydrogels are water-swellable orwater-swollen polymeric materials typically having structures definedeither by a crosslinked or an interpenetrating network of hydrophilichomopolymers or copolymers. In the case of physical crosslinking, thelinkages may take the form of entanglements, crystallites, orhydrogen-bonded structures to provide structure and physical integrityto the polymeric network.

Suitable hydrogels may be formulated from a variety of hydrophilicpolymers and copolymers including polyvinyl alcohol, polyethyleneglycol, polyvinyl pyrrolidone, polyethylene oxide, polyacrylamide,polyurethane, polyethylene oxide-based polyurethane, andpolyhydroxyethyl methacrylate, and copolymers and mixtures of theforegoing.

Silicone-base hydrogels are also suitable. Silicone hydrogels may beprepared by polymerizing a mixture of monomers including at least onesilicone-containing monomer and or oligomer and at least one hydrophilicco-monomer such as N-vinyl pyrrolidone (NVP), N-vinylacetamide,N-vinyl-N-methyl acetamide, N-vinyl-N-ethyl acetamide, N-vinylformamide,N-vinyl-N-ethyl formamide, N-vinylformamide, 2-hydroxyethyl-vinylcarbonate, and 2-hydroxyethyl-vinyl carbamate (beta-alanine).

The annulus may comprise an elastomer, such as those discussed justabove, reinforced with a fiber.

The fiber may be wrapped around the core member in a variety ofdifferent configurations, e.g., wrapping the core member in a randompattern, circumferential wrapping, radial wrapping, progressive polar(or near-polar) wrapping moving around the core, and combinations ofthese patterns and with other patterns.

The shape of each of the core members may be cylindrical, although theshape (as well as the materials making up the core member and the coremember size) may be varied to obtain desired physical or performanceproperties. For example, the core member's shape, size, and materialswill directly affect the degree of flexion, extension, lateral bending,and axial rotation of the prosthetic disc.

The annular capsule may be made of an appropriate polymer, such aspolyurethane or silicone or the materials discussed above, and may befabricated by injection molding, two-part component mixing, or dippingthe end plate-core-fiber assembly into a polymer solution. The annularcapsule may be oblong with straight sidewalls or with one or morebellows formed in the sidewalls. A function of the annular capsule is toact as a barrier that keeps the disc materials (e.g., fiber strands)within the body of the disc, and that keeps potential, natural in-growthoutside the disc.

Where a range of values is provided, it is understood that eachintervening value within the range, to the tenth of the unit of thelower limit (unless the context clearly dictates otherwise), between theupper and lower limit of that range and any other stated or interveningvalue in that stated range is described. The upper and lower limits ofthese smaller ranges may independently be included in the smaller rangesis also described, subject to any specifically excluded limit in thestated range. Where the stated range includes one or both of the limits,ranges excluding either or both of those included limits are alsodescribed.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe medical devices art. Although methods and materials similar orequivalent to those described here may also be used in the practice ortesting of the described devices and methods, the preferred methods andmaterials are described in this document. All publications mentionedherein are incorporated herein by reference to disclose and describe themethods and/or materials in connection with which the publications arecited.

It must be noted that as used herein and in the appended claims, thesingular forms “a”, “an”, and “the” include plural referents unless thecontext clearly dictates otherwise.

As will be apparent to those of skill in the art upon reading thisdisclosure, each of the individual variations described and illustratedherein has discrete components and features which may be readilyseparated from or combined with the features of any of the other severalembodiments without departing from the scope or spirit of thisdisclosure. For example, and without limitation, several of thevariations described here include descriptions of anchoring features,protective capsules, fiber windings, and protective covers coveringexposed fibers for integrated end plates. It is expressly contemplatedthat these features may be incorporated (or not) into those variationsin which they are not shown or described.

All patents, patent applications, and other publications mentionedherein are hereby incorporated herein by reference in their entireties.The patents, applications, and publications discussed herein areprovided solely for their disclosure prior to the filing date of thepresent application. Nothing herein is to be construed as an admissionthat contents of those patents, applications, and publications are“prior” as that term is used in the Patent Law.

The preceding merely illustrates the principles of the invention. Itwill be appreciated that those skilled in the art will be able to devisevarious arrangements which, although not explicitly described or shownherein, embody the principles otherwise described here and are includedwithin its spirit and scope. Furthermore, all examples and conditionallanguage recited herein are principally intended to aid the reader inunderstanding the described principles of my devices and methods.Moreover, all statements herein reciting principles, aspects, andvariation as well as specific examples thereof, are intended toencompass both structural and functional equivalents. Additionally, itis intended that such equivalents include both currently knownequivalents and equivalents developed in the future, i.e., any elementsdeveloped that perform the same function, regardless of structure.

1. A prosthetic intervertebral disc, comprising: a first end plate; asecond end plate; at least one compressible core member positionedbetween said first and second end plates; at least one fiber extendingbetween and engaged with said first and second end plates; and at leastone binding element compressing at least a portion of the disc to alower profile, and wherein said end plates and said core member are heldtogether by said at least one fiber.
 2. The prosthetic intervertebraldisc of claim 1 wherein the at least one binding element compresses oneend of the disc.
 3. The prosthetic intervertebral disc of claim 1wherein one binding element compresses one end of the disc.
 4. Theprosthetic intervertebral disc of claim 1 wherein two binding elementscompresses two ends of the disc.
 5. The prosthetic intervertebral discof claim 1 wherein the at least one binding element compresses both endsof the disc.
 6. The prosthetic intervertebral disc of claim 1 whereinthe at least one binding element is releasable by a slip knot.
 7. Theprosthetic intervertebral disc of claim 1 wherein the at least onebinding element is crimped and releasing the crimp releases the bindingelement.
 8. The prosthetic intervertebral disc of claim 1 wherein the atleast one binding element comprises a composition selected fromsynthetic polymers, natural polymers, metals, alloys, and coatedmaterials.
 9. The prosthetic intervertebral disc of claim 1 wherein theat least one binding element comprises a form selected frommonofilament, thread, multifilament, and cable.
 10. The prostheticintervertebral disc of claim 1 wherein the disc is bullet-shaped. 11.The prosthetic intervertebral disc of claim 1 wherein the disc islozenge-shaped.
 12. A kit for surgically replacing a discs in a spinewith a posterior approach, comprising exactly two of the prostheticdiscs of claim
 1. 13. The kit of claim 12 further comprising at leastone cannula suitable for a posterior approach configured to access adisc to be replaced and to bypass the spinal cord and local nerve rootsand further sized for passage of at least one of the two prostheticdiscs of claim
 1. 14. The kit of claim 12 wherein the first and secondend plates of each of the prosthetic discs have a length and a width,and wherein the length is greater than the width.
 15. The kit of claim14 wherein the first and second end plates of the prosthetic discs havea length:width aspect ratio of the first and second end plates is in therange of about 1.5:1 to 5.0:1.